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Aftertreatment system design involves multiple tradeoffs between engine performance, fuel economy, regulatory emission levels, packaging, and cost. Selection of the best design solution (or “architecture”) is often based on an assumption that inherent catalyst activity is unaffected by location within the system. However, this study acknowledges that catalyst activity can be significantly impacted by location in the system as a result of varying thermal exposure, and this in turn can impact the selection of an optimum system architecture. Vehicle experiments with catalysts aged over a range of mild to moderate to severe thermal conditions that accurately reflect select locations on a vehicle were conducted on a chassis dynamometer. The vehicle test data indicated CO and NOx could be minimized with a catalyst placed in an intermediate location.

Ignoring the impact of water condensation leads to incorrect temperature simulation during cold start, and this can lead to questions being raised about the overall accuracy of aftertreatment simulation tools for both temperature and emission predictions. This report provides a mathematical model to simulate the condensation and evaporation of water in exhaust after-treatment devices. The simulation results are compared with experimental data. Simulation results show that the temperature profiles obtained using the condensation model are more accurate than the profiles obtained without using the condensation model. The model will be very useful in addressing questions that concern the accuracy of the simulation tool during cold-start and heating up of catalysts, which accounts for the conditions where tailpipe emission issues are most significant.

Exhaust gas temperature is often measured with a device such as thermocouple or RTD (Resistance Temperature Detector). An alternative method to determine the gas temperature would be to use an existing gas sensor heating mechanism to perform as a temperature sensor. A planar type FLOH (Fast Light Off HEGO-Heated Exhausted Gas Oxygen) sensor under transient vehicle speed/load conditions is suited to this function and was modeled to predict the exhaust gas temperature. The numerical input to the model includes exhaust flow rate, heater voltage, and heater current. Laboratory experiments have been performed to produce an equation relating the resistance of the heater and the temperature of the sensor (heater), which provides a method to indirectly determine HEGO sensor temperature.

Thermocouples (TCs) are commonly used to measure exhaust gas temperature during automotive engineering experiments. To enhance the durability of TCs in the harsh exhaust gas environment, in many cases larger tip TCs (such as 1/8″ diameter) are used rather than smaller TCs. However, the signal from a larger thermocouple can differ significantly from that of small TC due to thermal capacitance of the tip, heat transfer to the exhaust pipe wall via conduction and radiation, and convection with exhaust gas. A model has been developed that calculates the effects of these factors and provides an estimate, for TCs of different sizes, of exhaust gas temperature. Experiments were performed to validate the model under transient (FTP) engine dynamometer conditions utilizing three popular TC sizes (1/32″, 1/16″, and 1/8″). Good correlation was found among predictions for various TC sizes.

A simple non-invasive thermocouple method is described and demonstrated that provides catalyst performance information. The thermocouple circuit consists of a Chromel wire attached to the stainless steel exhaust system before the catalyst and another Chromel wire after the catalyst. The exhaust system stainless steel functions as the other dissimilar metal component of a differential thermocouple. Measured electromotive force (EMF) between the thermocouple leads is proportional to the temperature difference across the catalyst and allows assessment of the performance of the catalyst between the thermocouple junctions. By measuring the difference directly, rather than measuring at two locations and using the difference between high temperatures at the two locations, one obtains a relatively accurate measurement even without calibration. A series of experiments were carried out to demonstrate the catalyst monitoring capabilities of this methodology.

Instrumentation of an exhaust system to measure surface temperature at multiple locations usually involves welding independent thermocouples to the surface of the system. This report describes a new type of thermocouple fabricated to measure temperature at a point or temperature difference between points on a metallic object utilizing the metal as one component of the new thermocouple. AISI 316 stainless steel is used in the current study to represent automotive exhaust pipe. The other component of the thermocouple is Nickel-Chromium (Chromel, Chromega), one of the two metals used in type K thermocouples, which are generally used for exhaust temperature measurements during emission tests. Use of the new thermocouple is contingent upon an accurate calibration of its response to changes in temperature.

Thermocouples are commonly used to measure exhaust gas temperature during automotive engineering experiments. In most cases, the raw measurements are used directly as an absolute indication of the actual exhaust gas temperature. However, in reality, the signal from a TC is only an indication of its own tip temperature. The TC indicated tip temperature can deviate significantly from the actual gas temperature due to factors such as thermal capacitance of the tip itself, and heat transfer to the exhaust pipe wall through conduction and radiation. A model has been developed that calculates the effects of these factors to provide an estimate of the actual exhaust gas temperature. Experiments were performed to validate the model under both transient and steady state engine dynamometer conditions utilizing three popular sizes of TCs. Good correlation among predictions for various TC sizes confirms the model's accuracy.

The optimized design of an exhaust emission system in terms of performance, cost, packaging, and engine control strategy will be a key part of competitively meeting future more stringent emission standards. Extensive use of vehicle experiments to evaluate design system tradeoffs is far too time consuming and expensive. Imperative to successfully meeting the challenges of future emission regulations and cost constraints is the development of an exhaust system simulation model which offers the ability to sort through major design alternatives quickly while assisting in the interpretation of experimental data. Previously, detailed catalyst models have been developed which require the specification of intricate kinetic mechanisms to determine overall catalyst performance. While yielding extremely valuable results, these models use complex numerical algorithms to solve multiple partial differential equations which are time consuming and occasionally numerically unstable.

The automotive industry and emission system suppliers invest considerable efforts for the improvement of the conversion efficiency of a catalytic converter, in order to lower vehicle emission. One of the methods to improve the catalyst conversion efficiency is to use a higher cell density brick with a thinner wall to increase its geometric surface area. However, there is a significant drawback for the system - higher pressure loss along the brick. Moreover, the mechanical strength and thermal degradation of the brick become major concerns. In this paper, the concept of a brick with radially variable cell density is introduced to possibly resolve several issues. A CFD study was conducted to verify benefits in both flow efficiency and pressure loss along the brick with several different flow rates.